I haven't used one because I prefer a digital multimeter, but they're popular with electricians as they're simple and rugged. From what I can tell, they're a solenoid and a spring -- the higher the voltage, the more deflection against the spring. You can feel and hear the solenoid being deflected -- important in a noisy environment.

It makes sense that it should beep then when it detects voltage, or when it's used to detect continuity without voltage.
It's just weird that it was beeping even when there's an open circuit.
It's used. I'm guessing it has gone bad. I never knew they made a meter that could do both at the same time, well, autosense what it should do.
Seems to me to be a bad design in that if it goofs by milliseconds, voltage is going to destroy the continuity testing part of the tester.
I'm guessing that that's what happened. Some of the other voltage testers will at least give a beep for one test and a chirp for the other. But not this one.
Ok, thanks for the input.